A student needed to estimate the concentration of reducing sugar in an unknown solution by using the Benedict’s test. Five reducing sugar solutions of different concentrations were supplied to help generate a calibration curve. The student added $2\ \text{cm}^3$ of Benedict’s solution to each reducing sugar solution, heated the mixtures in a water-bath and noted the time for the first colour change to appear. Which variables should the student keep constant while carrying out the Benedict’s test on each reducing sugar solution so that the results can be compared fairly?
- A1, 2 and 3
- B1 and 2 only
- C1 and 3 only
- D3 only